Directional diffusing film

ABSTRACT

A directional diffusing film of the invention includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film. A section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave. The directional diffusing film having characteristics specified by the invention can diffuse light in proper directionally diffused light, while maintaining high light-diffusing ability. The directional diffusing film utilizes light highly efficiently as compared with conventional diffusing films.

TECHNICAL FIELD

[0001] The present invention relates to a directional diffusing filmand, more particularly, to a directional diffusing film that can diffuselight in a controlled diffusion mode, a method of manufacturing thedirectional diffusing film, a surface light source unit and a liquidcrystal display.

BACKGROUND ART

[0002] Generally, a surface light source unit is provided with a lightdiffusing film. The light diffusing film is disposed on a light-emittingside of the surface light source unit to diffuse illuminating lightemitted by a light source of the surface light source unit.

[0003] A conventional light diffusing film is formed of a materialprepared by dispersing a light diffusing matter such as organic orinorganic beads in a transparent resin base, or is formed by coating atransparent resin base with an ink containing a dispersing matter suchas organic or inorganic beads.

[0004]FIG. 8 is a sectional view of a liquid crystal display 135provided with an edge-type surface light source unit 120 as an exampleof a conventional surface light source unit employing a conventionallight diffusing film.

[0005] As shown in FIG. 8, the surface light source unit 120 includes,as principal components, two light sources 121, a light guide plate 122,a reflecting film 124, and three light diffusing films 110-1, 110-2 and110-3 that have the same parameters.

[0006] The light guide plate 122 is a surface light emitting means andhas a light-emitting surface 122 a, a not light-emitting surfaceopposite the light-emitting surface 122 a, and opposite side surfaces.The light sources 121 are disposed beside the opposite side surfaces,respectively. A dot pattern 123 is formed on the not light-emittingsurface in order to diffuse light emitted by the light sources 121toward the light emitting surface 122 a. The reflecting film 124 isdisposed so as to face the not light-emitting surface of the light guideplate 122 to intercept light rays traveling in undesired directions andto reflect back the same in a predetermined direction.

[0007] The diffusing films 110-1, 110-2 and 110-3 disposed on the sideof the light emitting surface 122 a of the light guide plate 122 diffuselight emitted by the surface light source unit 120 to enhance theuniformity of light. The diffusing films 110-1, 110-2 and 110-3 concealthe dot pattern 123. A single diffusing film is unable to diffuse lightsatisfactorily and diffuses light irregularly. Usually, three diffusingplate or so are necessary to achieve satisfactory diffusion and toenhance front luminance.

[0008] A transparent liquid crystal display unit 133 is disposed on thelight emitting side of the surface light source unit 120. The liquidcrystal display unit 133 has a lower substrate 132, an upper substrate131 and a liquid crystal layer 130 sandwiched between the substrates 131and 132. In the liquid crystal display 135 shown in FIG. 8, the liquidcrystal display unit 133 is illuminated from behind by the surface lightsource unit 120.

[0009] The light diffusing films included in the aforesaid conventionalliquid crystal display contain beads as a diffusing matter. Therefore,light-diffusing characteristic of the light diffusing films isunsatisfactory if particle sizes of the beads are irregularlydistributed or if the beads are dispersed unsatisfactorily in a basematerial of the light diffusing films or a coating material of the lightdiffusing films. In addition, it is difficult to maintain the lightdiffusing films in a satisfactory appearance. Furthermore, productivity(yield) in manufacturing the light diffusing films is low.

[0010] In addition, it is possible that the beads fall off edges of thelight diffusing films when punching or cutting the light diffusing filminto a desired size or when assembling the punched or cut lightdiffusing films, which can produce dust and foreign matters.

[0011] The beads diffuse light in all directions. Consequently, theamount of light can be reduced by internal absorption and some part oflight can be reflected toward the light guide plate 122. Thus, theamount of light emitted in the normal (front) direction can be reduced,that is, efficiency of utilization of the light emitted by the lightsources 121 may be low, and luminance of the surface light source unit120 may be also low.

DISCLOSURE OF THE INVENTION

[0012] It is an object of the present invention to provide a directionaldiffusing film that can utilize light at a high efficiency and that canbe produced at a high productivity free from producing dust and foreignmatters, and a method of manufacturing such a directional diffusingfilm.

[0013] According to the present invention, a directional diffusing filmincludes a base film, and a plurality of convex directionality-providingelements regularly arranged on at least one surface of the base film;wherein a section of a first directionality-providing element in a firstplane including a top of the directionality-providing element has ashape of a substantially triangle, and a section of thedirectionality-providing element in a second plane including the top ofthe directionality-providing element and perpendicular to the firstplane has a top of a second directionality-providing element and a shapeof a continuous wave.

[0014] The directional diffusing film having characteristics specifiedby the present invention can diffuse light in proper directionallydiffused light, while maintaining high light-diffusing ability. Thedirectional diffusing film utilizes light highly efficiently as comparedwith conventional diffusing films.

[0015] Preferably, the shape of the continuous wave is a periodicwaveform having a period corresponding to each of thedirectionality-providing elements. For example, the periodic waveformmay be a substantially sinusoidal waveform.

[0016] The shape of continuous wave may be a periodic waveform havingwavy parts corresponding to the directionality-providing elements andstraight parts respectively extending between the adjacent wavy parts.

[0017] Preferably, the section of the directionality-providing elementin the first plane has a shape of a substantially isosceles triangle.Preferably, the substantially triangle has a vertex angle of a range of80° to 100°. Preferably, the substantially triangle has a vertex whichis rounded and/or a predetermined amount of which is cut off.

[0018] Preferably, the plurality of convex directionality-providingelements are arranged on a light emitting surface of the base film sothat incident light of incident angles in a range of 70° to 80° leavesthe directional diffusing film at outgoing angles including a maximumoutgoing angle in a range of 25° to 40°. In the case, light that isdifficult to be emitted out in prior art can be efficiently emitted outtoward the front direction.

[0019] Preferably, the plurality of convex directionality-providingelements are arranged on a light emitting surface of the base film sothat a haze value is in a range of 70 to 90 when light falls on a lightreceiving surface of the base film. Thus, luminous intensity (density)of diffused light in a necessary range can be increased.

[0020] According to the present invention, a method of manufacturing adirectional diffusing film having the aforesaid characteristicscomprises: a preparing step of preparing a cylinder having a moldingpattern of a shape complementary to the plurality of convexdirectionality providing elements; and a molding step of molding theplurality of convex directionality providing elements by introducing aresin into the molding pattern of the cylinder.

[0021] According to the present invention, the directional diffusingfilm having the aforesaid characteristics can be manufactured at amanufacturing cost comparable to that of the conventional diffusingfilm. In addition, shape-repeatability and productivity are improved.

[0022] Preferably, the molding step includes: a step of introducing anionizing radiation hardening resin into the molding pattern of thecylinder; and a step of setting the ionizing radiation hardening resinby irradiating ionizing radiation.

[0023] For example, the preparing step includes a step of forming themolding pattern of the shape complementary to the plurality of convexdirectionality providing elements by cell-engraving by means of anelectronic engraving machine provided with a diamond stylus having apoint angle in a range of 80° to 100°. Preferably, the diamond stylushas a tip that is rounded and /or cut by a predetermined amount.

[0024] In addition, this invention is a surface light unit comprising: alight source, a surface light emitting means having a light-emittingsurface that can emit a light from the light source in a predetermineddirection, and a directional diffusing film that includes a base film,and a plurality of convex directionality-providing elements regularlyarranged on at least one surface of the base film, wherein a section ofa first directionality-providing element in a first plane including atop of the directionality-providing element has a shape of asubstantially triangle, and a section of the directionality-providingelement in a second plane including the top of thedirectionality-providing element and perpendicular to the first planehas a top of a second directionality-providing element and a shape of acontinuous wave.

[0025] Alternately, this invention is a liquid crystal displaycomprising: a light source, a surface light emitting means having alight-emitting surface that can emit a light from the light source in apredetermined direction, a directional diffusing film that includes abase film, and a plurality of convex directionality-providing elementsregularly arranged on at least one surface of the base film, and atransparent liquid-crystal element arranged on a light-emitting side ofthe directional diffusing film, wherein a section of a firstdirectionality-providing element in a first plane including a top of thedirectionality-providing element has a shape of a substantiallytriangle, and a section of the directionality-providing element in asecond plane including the top of the directionality-providing elementand perpendicular to the first plane has a top of a seconddirectionality-providing element and a shape of a continuous wave.

[0026] The surface light source unit and the liquid crystal display areable to enhance luminance efficiently in a practical range of luminance,and the liquid crystal display is also capable of clearly displayingimages or the like, even if the number of the directional diffusingfilms is small. The surface light source unit and the liquid crystaldisplay can be formed in smaller thicknesses and can be sold at lowerprices, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1A is an enlarged, fragmentary perspective view of adirectional diffusing film in a preferred embodiment according to thepresent invention;

[0028]FIG. 1B is an enlarged, fragmentary perspective view of thedirectional diffusing film of FIG. 1A as seen in a direction of an arrowA in FIG. 1A;

[0029]FIG. 2A is a schematic plan view of the directional diffusing filmin the preferred embodiment;

[0030]FIG. 2B is a schematic sectional view taken along the line C-C(first plane) in FIG. 2A;

[0031]FIG. 2C is a schematic sectional view taken along the line D-D(second plane) in FIG. 2A;

[0032]FIG. 3 is a schematic view for explaining steps of forming convexdirectionality-providing elements included in the directional diffusingfilm of the embodiment;

[0033]FIG. 4 is a schematic view for explaining a method of forming aconcave pattern in a circumference of a cylinder;

[0034]FIG. 5 is a schematic sectional view of a liquid crystal displayprovided with a surface light source unit employing directionaldiffusing films of the embodiment;

[0035]FIG. 6 is a perspective view for explaining directions formeasurements of luminous intensity distribution;

[0036]FIG. 7A is a graph showing measured luminous intensitydistribution with respect to a direction perpendicular to the lamps;

[0037]FIG. 7B is a graph showing measured luminous intensitydistribution with respect to a direction parallel to the lamps; and

[0038]FIG. 8 is a schematic sectional view of a liquid crystal displayprovided with a surface light source unit employing conventional lightdiffusing films.

BEST MODE FOR CARRYING OUT THE INVENTION

[0039] Preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

Directional Diffusing Film

[0040]FIG. 1 is an enlarged, fragmentary perspective view of adirectional diffusing film 10 in a preferred embodiment according to thepresent invention. FIG. 1A is a perspective view seen in a direction ofan arrow B in FIG. 1B, and FIG. 1B is a perspective view seen in adirection of an arrow A in FIG. 1A.

[0041] The directional diffusing film 10 has a base film 11 and aplurality of directionality-providing elements 12. In FIG. 1, thedirectionality-providing elements 12 are magnified greatly forexaggeration. Actually, the convex directionality-providing elements 12are very small projections.

[0042] The base film 11 is formed of a transparent resin. The base film11 may be an oriented or nonoriented film of a thermoplastic resin, suchas cellulose triacetate, a polyester, polyamide, polyimide,polypropylene, poly(methyl pentene), poly(vinyl chloride), poly(vinylaceta), poly(methyl methacrylate), polycarbonate or polyurethane.Although dependent on rigidity, it is preferable that a thickness of thebase film 11 is in a range of 50 to 200 μm, from a viewpoint ofprocessing and/or handling it. In addition, in view of firmly and stablybonding the convex directionality-providing elements 12 to the base film11, it is preferable to finish a surface of the base film 11 to whichthe convex directionality-providing elements 12 are to be bonded, by anadhesion improving process such as a corona discharge process.

[0043]FIGS. 2A, 2B and 2C are a plan view and sectional views of thedirectional diffusing film 10. FIG. 2A is a plan view taken from theside of the light emitting surface of the directional diffusing film 10,FIG. 2B shows a section CC included in a first plane and extending inthe direction of the arrow B, and FIG. 2C is a section DD included in asecond plane and extending in the direction of the arrow A. As shown inFIG. 2C, an outline 12 a of the section DD of the convexdirectionality-providing elements 12 included in the second plane isgently curved. The outline 12 a of the section DD of the convexdirectionality-providing elements 12 may have straight lines dependentlyon a method and/or a condition for forming a molding cylinder 88.

[0044] As shown in FIG. 2B, the outline of the section CC of the convexdirectionality-providing elements 12 included in the first plane andextending in the direction of the arrow B has a shape of isoscelestriangles each having a vertex angle α=90°. The first plane includingthe section CC includes tops of the convex directionality-providingelements 12. Shapes of sections of the convex directionality-providingelements 12 included in planes parallel to the first plane and notincluding the tops of the convex directionality-providing elements 12are substantially similar to and lower than the shape of the section CCof the convex directionality-providing elements 12 included in the firstplane and extending in the direction of the arrow B.

[0045] The shape of the section CC of the convexdirectionality-providing elements 12 included in the first plane andextending in the direction of the arrow B does not need to be that ofisosceles triangles and may be that of any triangles other thanisosceles triangles.

[0046] The outline 12 a of the section DD included in the second planeincluding the tops of the convex directionality-providing elements 12has a sinusoidal waveform. A three-dimensional shape of the convexdirectionality-providing elements 12 is followed by moving the sectionCC of isosceles triangles extending in the direction of the arrow Balong the substantially sinusoidal waveform.

[0047] Referring to FIGS. 2A, 2B and 2C, each of the convexdirectionality-providing elements 12 has a vertex angle a in a range of80° to 100°, a height H in a range of 15 to 120 μm and lengths L1 and L2of the bottom in a range of 40 to 200 μm. Spaces D1 and D2 betweenadjacent convex directionality-providing elements 12 are 15 μm or below,respectively. Thus, the convex directionality-providing elements 12 arearranged in the highest possible density. The values of the parametersL1, L2 and H are determined dependently on operating conditions of theelectronic engraving machine, which will be described later. If thevertex angle a is outside the aforesaid range, front luminance isgreatly reduced.

[0048] In the directional diffusing film 10 shown in FIG. 2, the lengthL1 and L2 are approximately equal to each other. However, the lengths L1and L2 need not be equal to each other and may be of any suitablevalues, respectively.

[0049] The tops of the convex directionality-providing elements 12 maybe rounded and/or cut by a predetermined amount. However, luminancetends to be reduced when the tops are rounded and/or cut by apredetermined amount.

[0050] The convex directionality-providing elements 12 are formed of amixture of an oligomer, such as (meth)acrylate (hereinafter acrylate andmethacrylate will be referred to inclusively as “(meth)acrylate” of apolyfunctional compound, such as any polyhydric alchol, or a prepolymer,and a comparatively large amount of a reactive diluent. Suitablediluents include monofunctional monomers, such as ethyl (meth)acrylate,ethylhexyl (meth)acrylate, styrene, vinyltoluene and N-vinylpyrrolidone,and multifunctional monomers, such as trimethylolpropanetri(meth)acrylate, hexanediol (meth) acrylate, tripropylene glycol di(meth) acrylate, diethylene glycol di(meth)acrylate pentaerythritoltri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, 1,6 hexane diol(meth)acrylate and neopentyl glycol di(meth)acrylate.

[0051] When the aforesaid mixture contains a photo-polymerizationinitiator, such as acetophenone, benzophenoe, Michler's benzoylbenzoate, α-amyloxime ester or thioxanthone, and a photosensitizer, suchas n-butylamine, triethylamine or tri-n-butylphosphine, the same can beused as an ultraviolet hardening resin.

[0052] The mixture may contain, as an ionizing radiation hardeningresin, an active organic silicon compound expressed by R_(m)Si(OR′)_(n),where R′ is an alkyl group having a carbon number in a range of 1 to 10,and m and n are integers meeting m+n=4. Concretely, the organic siliconcompound may be tetramethoxysilane, tetraethoxysilane,tetra-iso-propoxysilane, tetra-n-propoxysilane, tetra-n-butoxysilane,tetra-sec-butoxysilane, tetra-tert-butoxysilane, tetrapentaethoxysilane,tetrapenta-iso-propoxysilane, tetrapenta-n-propoxy-silane,tetrapenta-n-butoxysilane, tetrapenta-sec-butoxysilane,tetrapenta-tert-butoxysilane, methyl trimethoxysilane, methyltriethoxysilane, methyl tripropoxysilane, methyl tributoxysilane,dimethyl dimethoxysilaane, dimethyl diethoxysilane, dimethylethoxysilane, dimethyl methoxysilane, dimethyl propoxysilane, dimethylbutoxysilane, methyl dimethoxysilane, methyl diethoxysilane, or hexyltrimethoxysilane.

[0053] The convex directionality-providing elements 12 may be formed ofa thermoplastic resin instead of the aforesaid reaction hardening resin.Possible thermoplastic resins are, for instance, acrylic resins, such asmethyl methacrylate and ethyl methacrylate, polyester resins, such aspolyethylene terephthalate, polybutylene terephthalate and polyethylenenaphthalate, polycarbonate resins, polyhydrocarbon resins, such aspolystyrene, polypropylene and polymethylpentene, polyamide resins, suchas nylon 66 and nylon 6, saponified ethylene-vinyl acetate copolymers,polyimide resins, polysulfone resins, polyvinyl chloride resins, andcellulose acetate resins.

[0054] In this embodiment, the base film 11 and the convexdirectionality-providing elements 12 are formed of the followingmaterials.

[0055] The base film 11 is a PET film with a thickness t=100 μm (A4300,commercially available from Toyobo).

[0056] The convex directionality-providing elements 12 are formed of anultraviolet hardening resin (RC19-793, commercially available from DaiNippon Inki Kagaku Kogyo K.K.).

Method of Manufacturing Directional Diffusing Film

[0057] The directional diffusing film 10 was made by forming the convexdirectionality-providing elements 12 on the base film 11.

[0058] Referring to FIG. 3, the molding cylinder 88 is provided in itscircumference with cavities (a concave pattern) 88 a having a shapecomplementary to that of the convex directionality-providing elements12, in a regular arrangement corresponding to that of the convexdirectionality-providing elements 12 of the directional diffusing film10. An ionizing radiation hardening resin 82 is fed to a die head 86 bya pump 87 and is extruded through the die head 86 evenly into thecavities 88 a. The base film 11 is pressed closely against thecircumference of the molding cylinder 88 by means of an inlet nip roller83. The ionizing radiation hardening resin 82 filling up the cavities 88a is irradiated through the base film 11 with ionizing radiationradiated by an ionizing radiation irradiation unit 85 (D-valve UV lamp,commercially available from Fusion) to set the ionizing radiationhardening resin 82 into a cured (set) resin 81 and to bond the curedresin 81 to the base film 11. Then, the convex directionality-providingelements 12 integrally combined with the base film 11 are separated fromthe molding cylinder 88 by means of an outlet nip roller 84 in order toobtain the directional diffusing film 10.

[0059]FIG. 4 is a view of assistance in explaining a method of formingthe cavities 88 a in the circumference of the molding cylinder 88 in anarrangement corresponding to the regular arrangement of the convexdirectionality-providing elements 12. In FIG. 4, directions indicated byarrows A and B correspond to those indicated by arrows A and B in FIGS.1 and 2, respectively.

[0060] The cavities 88 a were formed by a cell-engraving process using agravure electronic engraving machine (commercially available fromHeidelberg Japan) provided with a double-negative diamond stylus havinga point angle α=90°.

[0061] A diamond stylus 90 is supported on a fulcrum 90 a for minuteoscillation at a frequency of the order of several thousands hertz. Anamplitude of the minute oscillation is controlled according to arotating speed of the molding cylinder 88 in such a manner that theamplitude traces a substantially sinusoidal waveform. Thus, the cavities88 a are formed correspondingly to the convex directionality-providingelements 12 arranged in a substantially sinusoidal waveform. In theembodiment, the tip of the diamond stylus 90 was cut by 5 μm beforehandin order to prevent breakage thereof during the engraving process. Thecavities 88 a were formed on the molding cylinder 88 in 100 lines/cm andat an angle of 4°.

Surface Light Source Unit and Liquid Crystal Display

[0062]FIG. 5 is a sectional view of a liquid crystal display 35 providedwith a surface light source unit 20 employing the directional diffusingfilms 10 of the embodiment.

[0063] As shown in FIG. 5, the surface light source unit 20 includes twolight sources 21, a light guide plate 22, a reflecting film 24 and twodirectional diffusing films 10A and 10B. The directional diffusing films10A and 10B are the foregoing directional diffusing films 10 and havethe same parameters. The directional diffusing films 10A and 10B aredisposed in such a manner that four sides of each bottom of the convexdirectionality-providing elements 12 thereof (FIG. 2) extend at an angleof about 45° respectively with respect to four sides of a light emittingsurface 22 a of the rectangular light guide plate 22 (with respect to adirection perpendicular to the lamp and a direction parallel to thelamp).

[0064] The light guide plate 22, i.e., a surface light projecting means,has the light-emitting surface 22 a, a not light-emitting surfaceopposite the light-emitting surface 22 a, and opposite side surfaces.The light sources 21 are disposed near the opposite side surfaces,respectively. The not light-emitting surface is provided with a dotpattern 23 in order to diffuse light emitted by the light sources 21toward the light emitting surface 22 a. The reflecting film 24 faces thenot light-emitting surface of the light guide plate 22 to interceptlight rays traveling in unnecessary directions and to reflect back thelight rays in a predetermined direction.

[0065] A transparent liquid crystal display unit 33 is disposed on thelight emitting side of the surface light source unit 20. The liquidcrystal display unit 33 includes a lower substrate 32, an uppersubstrate 31, and a liquid crystal layer 30 sandwiched between the uppersubstrate 31 and the lower substrate 32. As shown in FIG. 5, the surfacelight source unit 20 illuminates the liquid crystal display unit 33 fromthe back side of the same.

Performance Evaluation Tests

[0066] The directional diffusing film 10 and the surface light sourceunit 20 including the directional diffusing film 10 were evaluated interms of front luminance, particle falling, and luminous intensitydistribution in comparison with a conventional directional diffusingfilm and a conventional surface light source unit including theconventional directional diffusing film.

[0067] Herein, the conventional surface light source (comparativeexample) is the surface light source unit 120 explained in connectionwith FIG. 8, which employs directional diffusing films D121 commerciallyavailable from Tujiden as the conventional directional diffusing films110-1, 110-2 and 110-3.

[0068] A haze value, which is a ratio of luminance of an object asobserved through a diffusing medium to that of the object as observeddirectly, was used as an index of a level of light diffusion. Thedirectional diffusing film 10 of the embodiment had a satisfactory hazevalue in the range of 70 to 90.

[0069] More concretely, the light sources 21 and 121 were turned on andthe front luminances of the surface light source units 20 and 120 weremeasured from a direction normal to the front surfaces of the respectivesurface light source units 20 and 120 by a luminance meter (BM-7, angleof field: 2°, made by Topcon).

[0070] Particle falling was evaluated by the number of foreign mattersfound when cutting the directional diffusing films in a predeterminedsize and incorporating the cut films into the surface light sourceunits.

[0071] Results of measurement of the front luminance and particlefalling are shown in Table 1. TABLE 1 FRONT LUMINANCE PARTICLE FALLINGEMBODIMENT 1495 cd/m² (102.1%) ◯ (NOT FOUND) COMPARATIVE 1465 cd/m²(100.0%) X (FOUND)

[0072] The front luminance of the surface light source unit of theembodiment was greater than that of the comparative example by about 2%.In addition, as resin fragments and fine particles fell from thecomparative example, but nothing fell at all from the surface lightsource unit of the embodiment.

[0073] Luminous intensities of the surface light source units 20 and 120with respect to two perpendicular directions were measured in theangular range of −80° to 80° at angular intervals of 1° by the luminancemeter (BM-7, angle of field: 20, made by Topcon)

[0074]FIG. 6 shows the directions for the measurement of luminousintensity distribution characteristics. In FIG. 6, a vertical directionperpendicular to the respective light sources (lamps) 21 is indicated byan arrow V and a horizontal direction perpendicular to the verticaldirection is indicted by an arrow H. Luminous intensity distributioncharacteristics were measured with respect to those two directions.

[0075]FIG. 7A is a graph showing the luminous intensity distributioncharacteristics with respect to the vertical direction perpendicular tothe lamps, and FIG. 7B is a graph showing the luminous intensitydistribution characteristics with respect to the horizontal directionparallel to the lamps.

[0076] As shown in FIG. 7A, the luminance of the surface light sourceunit 20 of the embodiment with respect to the vertical direction ishigher than that of the comparative example in the range of ±40° withrespect to the front (axial) direction (0° direction).

[0077] As shown in FIG. 7B, the luminance of the surface light sourceunit 20 of the embodiment with respect to the horizontal direction ishigher than that of the comparative example in substantially all theangular range.

[0078] As described above, the embodiment can achieve a high lightdiffusing efficiency and can diffuse light directionally such thatluminous intensity (density) is high in a necessary range. Thus, thesurface light source unit provided with directional diffusing films thenumber of which smaller than that of the conventional surface lightsource unit can have a luminous intensity in a necessary range higherand evener than that of the conventional surface light source unit.

[0079] Accordingly, a liquid crystal display employing the surface lightsource unit can be formed in a thickness smaller than that of theconventional liquid crystal display while capable of displaying clearimages or the like. In addition, the liquid crystal display can bemanufactured at a lower cost and be purchased at a lower price, comparedwith the conventional liquid crystal display.

Modifications

[0080] The present invention is not limited to the foregoing embodimentsspecifically described above, and many changes and variations may bemade therein without departing from the scope of the invention.

[0081] (1) Although the surface light source unit 20 in the foregoingembodiment is provided with the two directional diffusing films 10, asurface light source unit according to the present invention may beprovided with a single directional diffusing film or three or moredirectional diffusing films.

[0082] (2) Although the surface light source unit 20 in the foregoingembodiment is provided with the two directional diffusing films 10 abovethe light guide plate 22, a polarizing-separating film may be used incombination with the surface light source unit 20.

[0083] (3) Although the directionality-providing elements 12 of theforegoing embodiment have a shape that can be traced by moving a shapeof successive isosceles triangles along the substantially sinusoidalwaveform, the convex directionality-providing elements may have a shapethat can be traced by moving a shape of successive isosceles trianglesalong a shape of successive isosceles triangles.

[0084] (4) Although the directional diffusing films 10A and 10B aredisposed in such a manner that the four sides of each bottom of thedirectionality-providing elements 12 thereof extend at the angle ofabout 4° respectively with respect to the vertical direction and thehorizontal direction, the directional diffusing films 10A and 10B may bedisposed in such a manner that the four sides of each bottom of thedirectionality-providing elements 12 thereof extend in parallel to thevertical direction or the horizontal direction.

[0085] (5) Although all the convex directionality-providing elements 12of the foregoing embodiment have the same size, light diffusing mode maybe freely changed by forming different types of directionality-providingelements. For example, directionality-providing elements 12 farther fromthe light source 21 may be formed in sizes greater or smaller than thoseof the directionality-providing elements 12 nearer to the light source21.

[0086] (6) Although the bottoms of the directional diffusing pats 12 ofthe foregoing embodiment are substantially square, the bottoms of thesame may be rhombic or the lengths L1 and L2 may be different from eachother.

1. A directional diffusing film comprising: a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film; wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave.
 2. A directional diffusing film according to claim 1 , wherein the shape of the continuous wave is a periodic waveform having a period corresponding to each of the first and second directionality-providing elements.
 3. A directional diffusing film according to claim 2 , wherein the periodic waveform is a substantially sinusoidal waveform.
 4. A directional diffusing film according to claim 1 , wherein the shape of the continuous wave is a periodic waveform having wavy parts corresponding to the directionality-providing elements and straight parts respectively extending between the adjacent wavy parts.
 5. A directional diffusing film according to claim 1 , wherein the section of the first directionality-providing element in the first plane has a shape of a substantially isosceles triangle.
 6. A directional diffusing film according to claim 1 , wherein the substantially triangle has a vertex angle of a range of 80° to 100°.
 7. A directional diffusing film according to claim 1 , wherein the substantially triangle has a vertex which is rounded and/or a predetermined amount of which is cut off.
 8. A directional diffusing film according to claim 1 , wherein the plurality of convex directionality-providing elements are arranged on a light emitting surface of the base film so that incident light falling on the directional diffusing film at incident angles in a range of 70° to 80° leaves the directional diffusing film at outgoing angles including a maximum outgoing angle in a range of 25° to 40°.
 9. A directional diffusing film according to claim 1 wherein the plurality of convex directionality-providing elements are arranged on a light emitting surface of the base film so that a haze value is in a range of 70 to 90 when light falls on a light receiving surface of the base film.
 10. A method of manufacturing a directional diffusing film comprising: a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film; wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave, said method comprising: a preparing step of preparing a cylinder having a molding pattern of a shape complementary to the plurality of convex directionality-providing elements; and a molding step of molding the plurality of convex directionality-providing elements by introducing a resin into the molding pattern of the cylinder.
 11. A method according to claim 10 , wherein the molding step includes: a step of introducing an ionizing radiation hardening resin into the molding pattern of the cylinder, and a step of setting the ionizing radiation hardening resin by irradiating ionizing radiation.
 12. A method according to claim 10 , wherein the preparing step includes a step of forming the molding pattern of the shape complementary to the plurality of convex directionality-providing elements by cell-engraving by means of an electronic engraving machine provided with a diamond stylus having a point angle in a range of 80° to 100°.
 13. A method according to claim 12 , wherein the diamond stylus has a tip that is rounded and /or cut by a predetermined amount.
 14. A surface light source unit comprising: a light source, a surface light emitting means having a light-emitting surface that can emit a light from the light source in a predetermined direction, and a directional diffusing film that includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film, wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave.
 15. A liquid crystal display comprising: a light source, a surface light emitting means having a light-emitting surface that can emit a light from the light source in a predetermined direction, a directional diffusing film that includes a base film, and a plurality of convex directionality-providing elements regularly arranged on at least one surface of the base film, and a transparent liquid-crystal element arranged on a light-emitting side of the directional diffusing film, wherein a section of a first directionality-providing element in a first plane including a top of the directionality-providing element has a shape of a substantially triangle, and a section of the directionality-providing element in a second plane including the top of the directionality-providing element and perpendicular to the first plane has a top of a second directionality-providing element and a shape of a continuous wave. 